During President Trump’s medical exam this week, he was given a cognitive test and passed with a perfect score.

“I’ve found no reason whatsoever to think the president has any issues whatsoever with his thought processes,” said the president’s physician, Dr. Ronny L. Jackson, a rear admiral in the Navy.

The test, called the Montreal Cognitive Assessment, or Moca, is a 10-minute screening exam meant to highlight possible problems with thinking and memory. But it is by no means definitive, nor even diagnostic, experts pointed out.

Screening tests like these cannot rule out declines in reasoning or memory, or difficulties with planning or judgment. The test is just too blunt an instrument, and for many high-functioning people, too easy.

“You wouldn’t make a diagnosis in either direction based on a screening exam,” said Dr. Ronald Petersen, director of the Alzheimer’s Disease Research Center at the Mayo Clinic in Rochester, Minn. (He emphasized that he was speaking in general terms, not specifically about President Trump’s case.)

Here are some answers to questions about cognitive exams; what they measure, and how specialists decide whether a patient really is impaired.

This screening test was designed about 20 years ago as a possible replacement for another test, the Mini-Mental State Examination, which had been widely used since the 1970s to look for outright dementia. The Moca is used in all 31 of the National Institute on Aging’s Alzheimer Disease Centers.

While there are many such screening tests, the Moca is gaining acceptance because it is a bit harder than the Mini-Mental and can pick up problems that occur in the earliest stage of dementia, mild cognitive impairment — a sort of everyday forgetfulness.

About one in five people over age 65 have M.C.I., and roughly a third will develop Alzheimer’s disease within five years.

Moca has 30 questions meant to briefly assess memory, attention and concentration, control and self-regulation, and other mental skills.

To test memory, for example, the examiner reads five words at a rate of one per second and asks the subject to repeat them immediately and then again after some time has passed.

To assess attention and concentration, subjects are read a list of five digits and asked to repeat them in the order they were provided and then in reverse order. The subjects also are asked to count backward from 100 in increments of 7.

Other exercises include drawing a clock with the hands pointing to 11:10, and identifying a lion, rhino or camel. A perfect score is 30. A score from 26 to 30 is considered normal.

No. Such exams are not like mammograms for breast cancer and colonoscopies for colon cancer. With those tests, doctors can get a diagnosis and begin treatment.

Cognitive screenings like the Moca do not provide a specific diagnosis. And for most forms of cognitive decline, including Alzheimer’s disease, there is no effective treatment.

Nonetheless, Medicare recipients are often given cognitive screenings, noted Dr. Jason Karlawish, a dementia researcher at the University of Pennsylvania’s Perelman School of Medicine. That is because Congress instituted a requirement that Medicare cover a brief cognitive screening test as part of the annual wellness exam.

Maybe. But the test is not that difficult, and the problem with a single test is that the doctor doesn’t know what the subject’s starting point was. Usually it’s the trend over time that suggests a problem.

“If you are a Harvard professor, you could have a lot of decline before it starts to show up in cognitive tests,” said Dr. Eric Siemers, who until recently headed the development of Alzheimer’s drugs at Eli Lilly.

It’s not easy. Doctors start simply by talking to the patient: Has she noticed memory problems, or issues with judgment or reasoning?

It’s also important for the physician to talk to someone who knows the patient well, because people who are slipping cognitively do not always recognize it. “Lack of awareness or insight can be part of the package” of dementia, Dr. Petersen said.

If the doctor is concerned, and if a family member also says the subject is forgetful or repeating himself, and if this behavior is becoming a pattern — all those factors will influence the decision to “pursue this to the next level,” Dr. Petersen said.

Some patients simply prefer not to know if they are developing dementia. But those who do are given a neuropsychological test much more difficult and intense than the Moca. And doctors will repeat it over time.

In such a test, for example, the examiner reads a short story and asks the subject to repeat it. Thirty minutes later, the subject is asked to repeat the story again.

The subject is also asked to draw geometric shapes and to remember them a half-hour later. The examiner may ask the subject to recall a list of 15 words as many as five times, and then recall them 30 minutes later.

What physicians look for is a slow decline. At Washington University, for instance, researchers are studying people in families with an inherited form of Alzheimer’s disease. Those subjects come in annually for their tests; the initial score is much less important than a pattern of deterioration.

The results of neuropsychological tests can tell doctors how a subject is performing relative to others of the same age, sex and education level. If the doctor thinks something is amiss, a clinical exam might follow to figure out what might be causing the problem.

Most cases of dementia result from Alzheimer’s disease. An M.R.I. scan can help with diagnosis. It can detect a stroke and other conditions. It also can determine if the hippocampus, the memory center of the brain, is shrinking, as happens in Alzheimer’s.

A PET scan that uses glucose measures the activity of brain cells. Cells starting to falter and die, especially in certain areas of the brain, may be a sign of Alzheimer’s disease.